Servo writer and servo writing process

ABSTRACT

A servo writer according to the present invention is a servo writer which writes a servo signal on a servo band of a magnetic tape. It comprises a magnetic tape driving system in which a magnetic tape supplied from a supply reel is taken up by a take-up reel to drive the magnetic tape; a DC erasing head which is slidably in contact with said driving tape and which DC-magnetizes said servo band with imparting an azimuth relative to said servo band; and a servo signal writing head provided on a downstream of said DC erasing head in the driving direction of the magnetic tape, which is slidably in contact with said driving tape and which magnetizes the servo band by a magnetizing force having a component with a reverse magnetizing force relative to the magnetizing force of the DC erasing head in the lengthwise direction of the magnetic tape.

BACKGROUND ARTS

1. Field of the Invention

The present invention relates to a servo writer and a servo writingprocess, and particularly to a servo writer and a servo writing process,which can magnetize a servo band of a magnetic tape depending upon thedegree of the thickness of a magnetic layer, disclosure of which isbased on Japanese Patent Application Number 2003-296343, filed on Aug.20, 2003, which is incorporated herein by reference.

2. Description of Related Arts

In recent years, a magnetic recording medium has increasingly be able torecorded at higher density, for example, there exists a recording mediumfor backing up data for computer has a capacity as high as 100Gigabytes. For this reason, several hundreds of data tracks are formedin a width direction in the magnetic tape. In this connect, the width ofthe data track becomes very narrow accordingly, and a space betweenneighboring data tracks also becomes very narrow. For this reason, atechnique has been suggested in U.S. Pat. No. 5,689,384 where in orderto trace the recording/reading element of the magnetic head on the datatrack, a servo signal has been written on a magnetic tape in advance,the position of a magnetic head (the position of the magnetic tape inthe width direction) is servo-controlled, while reading the servo signalby the magnetic heat.

The servo signal as described above has hitherto been written on a servoband on a non-magnetized magnetic tape by applying a recording currentto the magnetic head of the servo writer so as to be magnetized in onedirection. Specifically, as shown in FIG. 7A, in order to record a servoband SS on the non-magnetized servo band SB, a recording pulse currentPC comprising a zero current and a plus pulse current has hitherto beenrun. When such recording pulse current PC is used, the magnetic tape MTis not magnetized only within the region of servo pattern SP in the caseof the zero current of the recording pulse current PC, and if the pluspulse current is run, the servo pattern SP is magnetized in onedirection through a leaked magnetic flux from a servo gap of themagnetic head. As a result, the servo signal SS is written. Data band DBon which data signal is written is between the servo bands SB and SB.

The servo signal SS is composed so that one servo pattern SP is formedby burst Ba, which is a magnetized portion in a state of two stripeseach having positive inclination angle relative to the driving direction(transferring direction), and the following burst Bb, which is amagnetized portion in a state of two stripes each having negativeinclination angle relative to the driving direction, and such servopatterns SP are repeatedly formed in the lengthwise direction atprescribed intervals. In this prior art, while the servo pattern SP iscomposed of two stripes each positively and negatively inclining in thiscase, the construction of the servo pattern SP may be suitably modified.For example, it may be composed of five stripes each positively andnegatively inclining, or five stripes each positively and negativelyinclining and four stripes each positively and negatively inclining maybe alternatively formed. For better understanding, the servo pattern SPis shown exaggeratedly relative to the magnetic tape MT in FIG. 7A.

On the other hand, in the device for recording/playing a magnetic tapethe change in the magnetic field of the servo signal SS is detected bythe change in the electric resistance through the servo signal readingelement (MR element), and is outputted as a differential waveform(voltage value) as the reading signal. For this reason, as the change inthe electric resistance of the MR element becomes large, the peakvoltage value of the reading signal of the servo signal SS becomeslarge, enhancing an S/N ratio of the reading signal. Consequently, inthe case where the change in the servo signal SS itself is large or inthe case where the region to be read is large due to wide width of theservo signal reading element (MR element), the peak voltage value of thereading signal RSL of the servo signal becomes large as shown in FIG.7C.

In the magnetic recording medium, it is expected that the recordingcapacity per one cartridge becomes much higher dense up to severalterabytes. Consequently, the number of the data tracks in the magnetictape is increased, the space between the neighboring data tracks becomesmuch more narrower, and the thickness of the magnetic tape becomesthinner. This decreases the magnetic amount, which can be detected atthe time of reading the servo signal, and reduces the change in themagnetic amount of the servo signal SS, which can be detected by theservo signal reading element. Consequently, as shown in FIG. 7D, thepeak voltage value of the reading signal RSS of the servo signal SSbecomes small, and the S/N ratio of the reading signal RSS isdeteriorated. As a result, the recording/playing device of the magnetictape cannot read the servo signal SS in a precise manner, and highlyprecise control of the position cannot be performed.

An object of the present invention is to provide a servo-writingprocess, which can magnetize a servo band of a magnetic tape dependingupon the degree of the thickness of a magnetic layer.

SUMMARY OF THE INVENTION

A servo writer according to the present invention is a servo writerwhich writes a servo signal on a servo band of a magnetic tape,comprising a magnetic tape driving system in which a magnetic tapesupplied from a supply reel is taken up by a take-up reel to drive themagnetic tape; a DC erasing head which is slidably in contact with saiddriving tape and which DC-magnetizes said servo band with imparting anazimuth relative to said servo band; and a servo signal writing headprovided on a downstream of said DC erasing head in the drivingdirection of the magnetic tape, which is slidably in contact with saiddriving tape and which magnetizes the servo band by a magnetizing forcehaving a component with a reverse magnetizing force relative to themagnetizing force of the DC erasing head in the lengthwise direction ofthe magnetic tape.

The present invention is also directed to a servo writing process, whichwrites a servo signal on a servo band of the magnetic tape, comprising:

-   -   DC-magnetizing a servo band by a DC erasing head having an        azimuth relative to the lengthwise direction of said magnetic        tape, and then    -   magnetizing said servo band through a magnetizing force        possessing a component with a reverse magnetizing force relative        to the magnetizing force of the DC erasing head in the        lengthwise direction of the magnetic tape to write the servo        signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a constructional view schematically showing the firstembodiment of the servo writer according to the present invention.

FIG. 2 is a plane view showing a servo signal writing head and DCerasing head included in the servo writer.

FIG. 3 is an enlarged view of FIG. 2.

FIG. 4 is a drawing showing the situation that the azimuth of the DCerasing head is changed.

FIG. 5A is a partially enlarged drawing of FIG. 4, and FIG. 5B is aenlarged view of FIG. 5A which enlarges the portion sup rounded by abroken line.

FIG. 6 is a constructional view schematically showing the secondembodiment of the servo writer according to the present invention.

FIG. 7 is an explanatory view showing the conventional magnetic tapehaving a servo signal, where FIG. 7A shows a recording current at thetime of writing the servo signal, FIG. 7B is a plane view of themagnetic tape, FIG. 7C is a drawing showing the recording signal of theservo signal where the recording element is wide, and FIG. 7D is adrawing showing the recording signal of the servo signal where therecording element is narrow.

FIG. 8A is an enlarged plane view showing the magnetization states ofthe magnetic tape according to one embodiment; FIG. 8B shows a servosignal read out from the magnetic tape of FIG. 8A, and FIG. 8C shows asignal at the time of writing the servo signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to attain the problem as described above, it can be consideredthat as suggested in our non-published patent application, after theservo band SB has been magnetized in one way in the lengthwise directionof the magnetic tape by a DC erasing head (not shown) (DCmagnetization), the servo signal SS is magnetized in the reversedirection to be recorded (see FIG. 8A). In FIG. 8A, the magnetizeddirection is shown as a small arrow. Since the output (peak voltagevalue) where the servo signal reading element reads the servo signal SSdepends upon the change ratio or change amount of switching between aportion where no signal is recorded and a portion where a signal isrecorded, the magnetism direction is greatly changed from the normaldirection to the reverse direction at the portion which switches fromthe base servo band SB having been magnetized in the normal directioninto the servo pattern SP having been magnetized in the reversedirection. For this reason, as shown in FIG. 8B, the servo signal SS canbe read out as a large output. Specifically, an S/N ratio of the servosignal SS can be enhanced.

However, since the degree of the thickness of the magnetic layerpossessed by the magnetic tape is greatly different from each otherdepending upon the kinds of the magnetic tapes, when the servo signal iswritten by the same servo writer by a method just mentioned, the playingcharacteristics of the servo signal unduly become different dependingupon the kinds of the products. For example, in the case of a thinnermagnetic tape, even if good playing characteristics of the servo signalcan be obtained, the playing characteristics of a thicker magnetic tapesometimes become poor. This is due to the fact that when the servo bandis magnetized by the DC erasing head (not shown) irreverent to thedegree of the thickness of the magnetic layer, the magnetization iscarried out utilizing a constant current value. It is consumed that theservo signal is written with the same current value.

It may be considered that by the magnetization depending upon thethickness of the magnetic layer may be performed controlling the currentvalue supplied to the DC erasing head. However, in this case, a newproblem that a circuit for supplying a current to the DC erasing headbecomes complicate is arisen.

Such a problem can be solved by a servo writer according to the presentinvention which writes a servo signal on a servo band of a magnetictape, comprising a magnetic tape driving system in which a magnetic tapesupplied from a supply reel is taken up by a take-up reel to drive themagnetic tape; a DC erasing head which is slidably in contact with saiddriving tape and which DC-magnetizes said servo band with imparting anazimuth relative to said servo band; and a servo signal writing headprovided on a downstream of said DC erasing head in the drivingdirection of the magnetic tape, which is slidably in contact with saiddriving tape and which magnetizes the servo band by a magnetizing forcehaving a component with a reverse magnetizing force relative to themagnetizing force of the DC erasing head in the lengthwise direction ofthe magnetic tape.

[First Embodiment]

The first embodiment of the servo writer according to the presentinvention will now be described by referring to the attached drawings.The servo writer according to the present invention is characterized inthat an azimuth of a DC erasing head contained in the servo writer isadjustable. The term “azimuth relative to the servo band” used hereinmeans an angle of magnetic gap of the magnetic head relative to theorthogonal direction of the servo band.

The construction of the servo writer according to the present inventionwill now be described. It is noted that the servo writer is a devicewhich is used in a stage for writing a servo signal onto a magnetictape.

A servo writer 10 shown in FIG. 10 mainly comprises a supply reel 11, atake-up reel 12, a device 13 for driving take-up reel, a servo signalwriting head 14, a DC erasing head 15, a pulse generating circuit 16, adevice 17 for moving DC erasing head, a controller 18, and a pluralityof guides 19. The servo writer also possesses a power device, averifying device for inspecting the servo signal written by the servosignal writing head 14 (not shown) and other devices. The supply reel11, the take-up reel 12, the device 13 for driving take-up reel and theguides 19 correspond to the magnetic tape driving system. The details ofeach portions of the servo writer 10 will be described.

In the supply reel 11, a magnetic tape MT cut into a product width fromthe wide web material wound around a pancake having a large diameter isset. The supply reel 11 supplies the magnetic tape MT at the time ofwriting a servo signal. The magnetic tape MT supplied from the supplyreel 11 is guided by the guides 19 to the servo signal writing head 14and the DC erasing head 15, after which it is taken up by the take-upreel 12. The supply reel 11 is rotatably driven by the device 13 fordriving take-up reel.

The device 13 for driving take-up reel is a device, which rotatablydrives the take-up reel 12, and comprises a motor, a circuit for drivingmotor, which supplies a current to the motor, a gear for connecting theshaft of the motor to the take-up reel 12 and the like (not shown). Thedevice 13 for driving take-up reel generates a motor current by thecircuit for driving motor based on a motor current signal inputted fromthe controller 18, supplies the motor current to the motor, andtransmits the rotational driving force to the take-up reel 12 via thegear to thereby rotatably drive the take-up reel.

The servo signal writing head 14 is a magnetic head for writing theservo signal SS onto the magnetic tape MT, and comprises a coil (notshown) for generating a magnetic flux, and four head gaps 14 a, 14 a, 14a, and 14 a aligned in one line corresponding to the positions of fourservo bands SB, SB, SB, SB, of the magnetic tape in the width direction(see FIG. 2). Head gaps 14 a are formed into non-parallel shape(none-connecting V-shape) having a prescribed angle relative to thelengthwise direction of the magnetic tape MT (driving direction). Thehead gaps 14 a are formed by a lithography to which a semiconductortechnique is applied. When a recording pulse current is supplied fromthe pulse generation circuit 16, the servo signal writing head 14magnetizes the magnetic layer of the magnetic tape MT by the leakagemagnetic flux from the head gaps 14 a to write the servo signal SS. Atthis time, the servo signal SS is written along with the drivingdirection of the magnetic tape MT.

The DC erasing head 15 is a magnetic head for magnetizing the magnetictape MT in the normal direction (DC magnetization), and is provided onan upstream of the servo signal writing head 14. The DC erasing head 15comprises a coil (not shown) for generating a magnetic flux, and fourhead gaps 15 a, 15 a, 15 a, and 15 a aligned in one line correspondingto the positions of four servo bands SB, SB, SB, SB, of the magnetictape in the width direction (see FIG. 2). Head gaps 15 a are formed intoan I shape in the width direction of the magnetic tape. The head gaps 15a are formed by a lithography to which a semiconductor technique isapplied. When a magnetism-erasing pulse signal is supplied from thepulse generation circuit 16, the DC erasing head 15 magnetizes themagnetic tape MT in the normal direction (driving direction of themagnetic tape MT) by the leakage magnetic flux from the head gaps 15 a(see FIG. 3).

The pulse generating circuit 16 is a circuit for supplying a recordingpulse current PC, which is a signal for writing a servo pattern to theservo signal writing head 14 at the time of writing the servo signal(see FIG. 8C). Also, the pulse generating circuit 16 is a circuit forcontinuously applying a magnetism erasing signal to the DC erasing head14 at the time of writing the servo signal.

Referring to FIG. 8C, the pulse generating circuit 16 repeats a patternwhere a plus pulse signal PP1, which is a plus polarity, zero currentZC1, plus pulse signal PP1, and zero current ZC1 are continuouslygenerated on this order, and then no current is generated for aprescribed period (zero current ZC1) to generate the recording pulsecurrent PC based on the pulse control signal inputted from thecontroller 18. Then, the pulse generating circuit 16 supplies therecording pulse current PC to the coil (not shown) of the servo signalwriting head 14.

The current value of the plus pulse current PP1 is a current sufficientfor magnetizing the magnetic layer of the magnetic tape MT by theleakage magnetic flux from the head gaps 14 a, and is set consideringthe characteristics of the coil of the servo signal writing head 14 andthe like. The width (period) of the plus pulse current PP1 may define aprescribed width of the servo pattern SP in the lengthwise direction,and is set considering the driving speed of the magnetic tape MT, shapeof the head gap 14 a and the like. The prescribed period of the zerocurrent ZC1 may define a prescribed interval for forming the servopattern SP described above, and is set considering the driving speed ofthe magnetic tape MT and the like.

The device 17 for moving DC erasing head is a device for changing theazimuth of the DC erasing head 15. In this embodiment, as the device 17for moving DC erasing head, a piezo-electric element is used. The device17 for moving DC erasing head changes the azimuth of the DC erasing head15 at the time of writing the servo signal SS based on the controlsignal of the head moving device (see FIG. 4). The term “azimuth”intended herein is an angle of the servo band SB of the magnetic tape MTand the head gap 15 a of the DC erasing head 15. Specifically, θ in FIG.4 is azimuth.

As shown in FIG. 4, when the azimuth of the DC erasing head 15 ischanged, the force for magnetizing the magnetic tape MT possessed byhead gap 15 a of the DC erasing head 15 (hereinafter referred to as“magnetizing force”) is decreased. Specifically, taking the originalmagnetizing force as P (see FIG. 5), in the case where the azimuth is θ,the component directed towards one direction of the lengthwise directionof the tape amongst the magnetizing force of the head gap 15 a is P×cosθ (see FIG. 5B). Specifically, when the azimuth is θ, the magnetizingforce of the head gap 15 a becomes lower than the case where the azimuthis 0°.

The controller 18 is a device for controlling actuation of each part ofthe servo writer 10, and comprises CUP (central processing unit),various storage devices, and the like. The controller 18 produces amotor current signal for controlling the motor current of the take-upreel 13 in order to keep the driving speed of the magnetic tape MT atthe time of writing the servo signal, which is sent to the device 13 fordriving take-up reel. The controller 18 also produces the magnetismerasing signal for magnetizing (or erasing the magnetism of) themagnetic layer of the magnetic tape MT at the time of writing the servosignal, which is sent to the DC erasing head 15.

In order to set the width of the servo pattern SP and the servo signalSS in which the prescribed interval for forming the servo pattern SP,the controller 18 produces a pulse control signal for controlling thecurrent value of the plus pulse current PPC of the recording pulsecurrent PC, the width of the pulse and the generation timing, which issent to the pulse generation circuit 16. Specifically, the controller 18produces the pulse pattern of the plus pulse current PP1→zero currentZC1→the plus pulse current PP1→zero current ZC1.

The controller 18 also produces a control signal for the device formoving DC erasing head in order to change the azimuth θ of the head gap15 a of the DC erasing had 15, which is sent to the device 17 for movingDC erasing head.

Next, a process for writing a servo signal onto the magnetic tape MTutilizing the servo writer 10 composed as described above (servo writingprocess) will now be described.

The magnetic tape MT in the pancake state is set on the supply reel 11of the servo writer 10, and the edge of the magnetic tape MT isconnected to the core of the take-up reel 12. The magnetic tape MT istaken up by the take-up reel 13 driven by the device 13 for drivingtake-up reel, while being guided by the guides 19 or such whereby themagnetic tape is run.

Next, the portion of the servo band SB of the magnetic tape MT ismagnetized by the DC erasing head 15 (see FIG. 3). At this time, bymeans of the device 17 for moving DC erasing head, the azimuth θ of theDC erasing had 15 is changed to adjust the magnetizing force. Asdescribed above, the azimuth of the DC erasing head 15 is changed by thedevice 17 for moving DC erasing head depending upon the degree of thethickness of the magnetic layer of the magnetic tape, whereby the servoband SB can be magnetized depending upon the degree of the thickness ofthe magnetic layer of the magnetic tape.

Then, the servo band SB having been magnetized in the normal directionis magnetized in the reverse direction by means of the pulse generatingcircuit 16 to write the servo signal SS (see FIG. 3). As a result, theservo pattern SP having been magnetized in the reverse direction isformed on the base servo band SB having been magnetized in the normaldirection of the magnetic tape MT.

The magnetic tape MT having the servo signal written thereon is taken upby the take-up reel 12 depending upon the specification of the product,and then stored in a cartridge case (not shown).

As described above, according to the servo writer 10, the azimuth of theDC erasing head 15 can be changed depending upon the thickness of themagnetic layer of the magnetic tape MT by the device 17 for moving DCerasing head, whereby the magnetizing force of the servo band SB bymeans of the DC erasing head can be adjusted. What is more, according tothe servo writer 10, since the magnetizing force of the servo band SB isadjusted by changing the azimuth of the DC erasing head 15, the currentvalue running through the DC erasing head 15 may be constant value.Consequently, there arises no problem in terms of complication of thecircuit for supplying current to the DC erasing head 15.

[Second Embodiment]

Second embodiment of the servo writer according to the present inventionwill now be described. Since this embodiment modifies the servo writeraccording to the first embodiment, the constitution elements similar tothose of the first embodiment are assigned to the same numerical orsymbols, the description thereof will be omitted.

As shown in FIG. 6, head gaps 15 b are formed on the DC erasing head sothat they project prescribed length on both ends of the magnetic tape MTin the width direction in order to cover the whole of the magnetic tapeMT in the width direction. Specifically, the head gaps 15 b are formedinto a length so as to cover each of the servo bands SB even if the DCerasing head 15 is rotated up to the maximum azimuth, which has beenpredetermined.

Between the DC erasing head 15 and the servo signal writing head 14, anAC erasing head 20, which AC magnetizes the data band DB positionedbetween the servo bands SB (magnetization at random in one direction andthe other direction of the lengthwise direction of the tape) isprovided. The AC erasing head 20 may be provided on a downstream of theDC erasing head 15, for example, may be provided on a downstream of theservo signal writing head 14.

Next, a process for writing a servo signal onto the magnetic tape MTutilizing the servo writer 10 composed as described above (servo writingprocess) will now be described.

First, the DC erasing head 15 is rotated by the device 17 for moving DCerasing head until the azimuth becomes a prescribed value. At this time,since the head gap 15 b of the DC erasing head 15 is formed in a lengthwhich covers each servo band SB even when the DC erasing head 15 isrotated to be the maximum azimuth, it can be securely in contact withthe whole of each servo band SB.

When the alignment of the DC erasing head 15 has been completed, themagnetic tape MT is driven by driving the device 13 for driving take-upreel and the whole of the magnetic tape MT is magnetized in the normaldirection by the DC erasing head 15. Subsequently, the data band DBhaving been magnetized in the normal direction is AC magnetized by theAC erasing head 20, and the servo band SB having been magnetized in thenormal direction is magnetized in the reverse direction by the servoband writing head 13 to write the servo signal SS.

According to the second embodiment described above, even if the DCerasing head 15 is largely rotated by the device 17 for moving DCerasing head, since head gap 15 b of the DC erasing head 15 is not outof the servo band SB, the azimuth can be greatly changed to largelychange the component of the magnetizing forcer in the lengthwisedirection of the tape.

While the embodiments of the present invention have been described, itshould be noted that the present invention is not restricted to theseembodiments and various modification and alternation can be made withoutdeparting from the sprits and the scope of the present invention.

For example, the recording current in the embodiments comprises a pulsecurrent in which pulse currents having a plus polarity and zero currentsare alternatively repeated, the pattern of the pulse current should notbe restricted thereto. For example, a pulse current in which pulsecurrents having a minus polarity and zero currents are alternativelyrepeated, the pattern of the pulse current may be used.

In these embodiments, whereas the base portion of the servo band ismagnetized in the normal direction and the portion of the servo signalis magnetized in the reverse direction, the base portion of the servoband may be magnetized in the reverse direction and the portion of theservo signal may be magnetized in the normal direction.

Also, whereas the embodiments are composed so that the magnetism of theservo signal writing head is reverse relative to the component of themagnetizing force of the magnetic tape in the lengthwise direction ofthe magnetic tape, the present invention is not restricted thereto. Themagnetizing force of the servo signal writing head may be any directionas long as it possesses a component which is the reverse directionrelative to the magnetizing force of the magnetic tape in the lengthwisedirection of the magnetic tape.

1. A servo writer, which writes a servo signal on a servo band of amagnetic tape, comprising a magnetic tape driving system in which amagnetic tape supplied from a supply reel is taken up by a take-up reelto drive the magnetic tape; a DC erasing head which is slidably incontact with said driving tape and which DC-magnetizes said servo bandwith imparting an azimuth relative to said servo band; and a servosignal writing head provided on a downstream of said DC erasing head inthe driving direction of the magnetic tape, which is slidably in contactwith said driving tape and which magnetizes the servo band by amagnetizing force having a component with a reverse magnetizing forcerelative to the magnetizing force of the DC erasing head in thelengthwise direction of the magnetic tape.
 2. The servo writer accordingto claim 1, which further comprises a device for moving DC erasing head,which changes the azimuth of the DC erasing head.
 3. The servo writeraccording to claim 1, wherein said head gaps of said DC erasing head areformed over the whole of the magnetic tape in the width direction, andwhich further comprises an AC erasing head for AC magnetizing the databand positioned between the servo bands provided on a downstream of theDC erasing head
 4. The servo writer according to claim 2, wherein saidhead gaps of said DC erasing head are formed over the whole of themagnetic tape in the width direction, and which further comprises an ACerasing head for AC magnetizing the data band positioned between theservo bands, provided on a downstream of the DC erasing head
 5. Theservo writer according to claim 1, wherein said servo signal writinghead comprises a coil for generating a magnetic flux, and a plurality ofhead gaps aligned in one line corresponding to the positions of saidplurality of servo bands of the magnetic tape in the width direction. 6.The servo writer according to claim 2, wherein said servo signal writinghead comprises a coil a coil for generating a magnetic flux, and aplurality of head gaps aligned in one line corresponding to thepositions of said plurality of servo bands of the magnetic tape in thewidth direction.
 7. The servo writer according to claim 1, wherein saidDC erasing head comprises a coil for generating a magnetic flux, and aplurality of head gaps in one line corresponding to the positions ofsaid plurality of servo bands of the magnetic tape in the widthdirection.
 8. The servo writer according to claim 2, wherein said DCerasing head comprises a coil for generating a magnetic flux, and aplurality of head gaps in one line corresponding to the positions ofsaid plurality of servo bands of the magnetic tape in the widthdirection.
 9. The servo writer according to claim 5, wherein said DCerasing head comprises a coil for generating a magnetic flux, and aplurality of head gaps in one line corresponding to the positions ofsaid plurality of servo bands of the magnetic tape in the widthdirection.
 10. The servo writer according to claim 6, wherein said DCerasing head comprises a coil for generating a magnetic flux, and aplurality of head gaps in one line corresponding to the positions ofsaid plurality of servo bands of the magnetic tape in the widthdirection.
 11. The servo writer according to claim 2, wherein saiddevice for moving DC erasing head utilizes a piezo-electric element. 12.The servo writer according to claim 4, wherein said device for moving DCerasing head utilizes a piezo-electric element.
 13. The servo writeraccording to claim 6, wherein said device for moving DC erasing headutilizes a piezo-electric element.
 14. The servo writer according toclaim 8, wherein said device for moving DC erasing head utilizes apiezo-electric element.
 15. The servo writer according to claim 10,wherein said device for moving DC erasing head utilizes a piezo-electricelement.
 16. The servo writer according to claim 1, which comprises apulse generation circuit which repeats a pattern where plus pulsecurrent, a zero current, a plus pulse current, a zero current arecontinuously generated, and then no current is generated.
 17. The servowriter according to claim 15, which comprises a pulse generation circuitwhich repeats a pattern where plus pulse current, a zero current, a pluspulse current, a zero current are continuously generated, and then nocurrent is generated.
 18. The servo writer according to claim 1, whichcomprises a pulse generation circuit which repeats a pattern where minuspulse current, a zero current, aminus pulse current, a zero current arecontinuously generated, and then no current is generated.
 19. Aservowriting process, which writes a servo signal on a servo band of themagnetic tape, comprising: DC-magnetizing a servo band by a DC erasinghead having an azimuth relative to the lengthwise direction of saidmagnetic tape, and then magnetizing said servo band through amagnetizing force possessing a component with a reverse magnetizingforce relative to the magnetizing force of the DC erasing head in thelengthwise direction of the magnetic tape to write the servo signal. 20.A servo writing process according to claim 19, wherein the whole of saidmagnetic tape is DC magnetized in DC magnetizing said servo band, andwhich further comprises AC magnetizing the data band between said servobands by an AC erasing head.